Conceivable methods of manufacturing a tube with longitudinally oriented internal holes include, for example, a method of machining holes in a rod-like member using a tool such as a drill. However, when the rod-like member is long and the holes to be formed are small in diameter, a long and thin drill bit is used. Whirl that occurs at the tip of the drill bit when such a drill bit rotates limits the performance of the machining of holes. To combat this, holes can be cut in rod-like members shorter than a predetermined length, and by joining these rod-like members with the holes in a rectilinear manner, a tube of a predetermined length with internal holes can be manufactured.
Examples of such a method of manufacturing a tube include the technology in the Patent Documents described below. Patent Document 1 describes a method of manufacturing a hydroformed tube, the method comprising friction stir welding an abutment portion of a plurality of cylindrical metal pipes in a coaxial arrangement, with a thick-walled cylindrical backing jig made of rigid material being disposed inside the abutment portion to support the abutment portion. The friction stir welding is performed by a rotating tool and a pin, which are integrally rotated at high speeds and inserted into the abutment portion, traversing the abutment portion in the circumferential direction. Patent Document 2 describes joining two columns by welding end portions of the two columns, wherein joining is performed after a first engagement portion of a column backing metal is inserted in a top end portion of a lower column and fixed, a bottom end portion of an upper column is fitted to the outside of a second engagement portion of the column backing metal and fixed, and the upper and lower columns are abutted, mediated by the column backing metal.